Signal processing system with baseband noise modulation and noise fold back reduction

ABSTRACT

A digital-to-analog converter (“DAC”) system utilizes chopping modulation technology to remove 1/f and other baseband noise from a baseband of a signal of interest. Chopping modulation and demodulation circuitry of the DAC operate at a chopping frequency equal to approximately one-half of a digital input signal sampling frequency. Chopping at one-half the sampling frequency allows fold back into the baseband of the input signal&#39;s frequency components and reduces fold back of noise, such as quantization noise, residing outside the baseband. In a further embodiment, a notch filter attenuates signals having frequencies around the chopping frequency prior to chopping to reduce fold back of noise into the baseband due to parasitic modulation. Coordination of chopping timing also reduces noises in the output of the DAC system.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Application No. 60/441,296, filed Jan. 21, 2003 andentitled “Segmented Chopping Amplifier”.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates in general to the field of signalprocessing, and, more specifically, to a system and method formodulating baseband noise and reducing noise that occurs, in part, dueto non-ideal system properties that mix noise into a baseband via foldback mechanisms.

[0004] 2. Description of the Related Art

[0005] Many electronic systems employ signal processing technology toprocess analog, digital, or a mix of analog and digital signals.Components used to implement signal processing technology often generateunwanted noise. For example, digital-to-analog converters (hereinafter,“DAC”) are widely used to convert digital signals into analog signals.In the process of conversion, DACs often generate noise through, forexample, quantization errors (“quantization noise”) and low frequency1/f noise.

[0006] In audio applications, the digital to analog conversion processoften involves oversampling a digital signal, modulating the signalusing a delta-sigma modulator to shape noise associated with the digitalsignal, and performing a digital to analog conversion using a low-passfilter. The filtered output signal is generally amplified to produce ananalog signal suitable for driving a load. Delta-sigma modulatorsreceive an input signal and convert the signal into a series of binarypulses having an average amplitude over time proportional to the inputsignal. In the process of producing a modulated output signal,delta-sigma modulators introduce quantization noise into the modulatedinput signal. However, the quantization noise advantageously residesoutside of the audio baseband where frequency components of interestreside, i.e. between about 0 Hz and above about 20-25 kHz. Nevertheless,some post modulation processing, such as a post-modulation digital toanalog conversion and low pass filtering, introduces noise into theaudio baseband.

[0007] One common type of noise generated in post modulation processingcircuits, such as metal oxide semiconductor gain stages, is 1/f noisewhich, as the nomenclature implies, has relatively high energy at lowfrequencies that rapidly diminishes at higher frequencies. Analogfilters often include one or more gain stages that introduce 1/f noise.A modulation technique referred to as “chopping” has been implemented inconventional technology to modulate 1/f noise out of the audio baseband.

[0008]FIG. 1 depicts a chopping circuitry and amplifier 100, which isutilized as a component in many well-known circuits such as switchedcapacitor digital to analog converters. The input signal, x₁(t) ismodulated by chopper circuit 102 at a chopper control signal c(t)frequency of, f_(chop). FIG. 2A depicts modulated input signal X₁(f) inthe frequency domain, centered f_(chop), and harmonics of f_(chop). Theamplitude of the modulated input signal X₁(f) decreases with 1/n, wheren is the harmonic number. The baseband of X₁(f) extends to frequencyf_(B), which in audio applications is about 20-25 kHz. The 1/f noise isadded to the modulated input signal x₁(t) after chopping.

[0009]FIG. 2B illustrates the 1/f noise in the frequency domain. Asmentioned above, the energy of the 1/f noise is primarily located withinlow frequencies, including the baseband of audio signals. Gain stage104, which may be part of a larger circuit (not shown), such as a lowpass filter, amplifies the modulated input signal x₁(t) and 1/f noise.

[0010] Chopper circuit 106 demodulates the output signal of gain stage102 at the frequency of chopper signal c(t) to produce output signalx₂(t). FIG. 2C depicts signal x₂(t) and the 1/f noise signal in thefrequency domain.

[0011] The demodulation of signal x₁(t) moves the output signal ofinterest, x₂(t), in the frequency domain back to the baseband andcenters the 1/f noise at f_(chop) and harmonics thereof, thus out of thebaseband. In an audio application, a low pass filter (not shown)attenuates signals having frequency components of x₂(t) outside f_(B).

[0012] U.S. Pat. No. 4,939,516 issued Jun. 3, 1990 and entitled “ChopperStabilized Delta-Sigma Analog-to-Digital Converter”, Early et alinventors (hereinafter, “Early”), and U.S. Pat. No. 5,039,989 issuedAug. 13, 1991 and entitled “Delta-Sigma Analog-to-Digital Converter withChopper Stabilization at the Sampling Frequency,” Welland et alinventors (hereinafter, “Welland”), describe conventional applicationsof chopping in analog-to-digital converters (hereinafter, “ADC”). Earlyand Welland proposed solutions using chopping circuitry to address 1/fand other noise issues that are particularly unique to ADCs.

[0013] Early proposed synchronizing a chopping frequency with an analoginput signal sampling frequency and a digital filter. Early selected achopping frequency equal to one-half of an analog input signal samplingfrequency so that the chopping frequency would be in the rejectionportion of the digital filter's frequency response. Early consideredsuch synchronization to be important because the digital filter was ableto provide a rejection of the 1/f noise that was modulated to thechopping frequency. See, for example, Early, col. 8, Ins. 37-64.

[0014] Welland recognized that, in an ADC, choosing a chopping frequencyequal to one-half of the sampling frequency of an analog input signalcan actually increase the modulation of noise into an input signal'sbaseband. Thus, Welland selected a chopping frequency equal to thesampling frequency of the analog input signal. Welland included ananalog modulator with at least one stage of amplification having afrequency response that provides a substantial amount of attenuation atthe sampling frequency of the analog modulator in the Welland design.Thus, the amplification stage attenuates 1/f noise, which is shifted upin frequency by chopping to the sampling frequency.

[0015] DACs are in many ways very different from ADCs. Consequently,chopping frequencies of DACs are selected for different reasons thanADCs. Conventional technologies implement chopping in DACs at a choppingfrequency that is as low as possible relative to a digital samplingfrequency. The DAC chopping frequency is conventionally chosen just highenough to shift 1/f noise out of the baseband of the input signal inorder to minimize parasitic effects associated with chopping circuitry.

SUMMARY OF THE INVENTION

[0016] In one embodiment of the present invention, a digital to analogconverter system includes a digital to analog converter (“DAC”) to (i)receive a digital input signal, sampled at frequency f_(S) and havingfrequency components primarily within a frequency baseband, and out ofbaseband noise and to (ii) convert the digital input signal into arepresentative analog signal. The DAC includes a modulation circuithaving a modulation frequency of approximately or equal to f_(S)/2 tomodulate the digital input signal by approximately or equal to f_(S)/2and a gain stage coupled to the modulation circuit to receive signalsmodulated by the modulation circuit and provide gain for the DAC,wherein during operation the gain stage is associated with noise havingfrequencies within the baseband. The DAC further includes a demodulationcircuit, coupled to the gain stage and having a demodulation frequencyequal to approximately or equal to f_(S)/2, to demodulate signals andallow frequency components of the input signal to fold back into thebaseband, to reduce fold back into the baseband of the quantizationnoise frequency components, and modulate the noise associated with thegain stage substantially out of the baseband.

[0017] In another embodiment of the present invention, a method ofconverting a digital input signal, sampled at a frequency f_(S) andhaving frequencies within a baseband, into a representative analogsignal, modulating out of the baseband low frequency noise componentsassociated with a digital to analog converter (“DAC”) gain stage, andreducing fold back of quantization noise, frequency components residingoutside the baseband, the method includes modulating a DAC input signal,which is derived from the digital input signal and the quantizationnoise by a modulation frequency of approximately or equal to f_(S)/2.The method further includes providing the modulated DAC input signal tothe DAC gain stage. The method also includes demodulating output signalsof the DAC gain stage by a demodulation frequency of approximately orequal to f_(S)/2 to allow frequency components of the input signal tofold back into the baseband, to prevent fold back into the baseband ofthe quantization noise frequency components, and modulate the noiseassociated with the gain stage substantially out of the baseband.

[0018] In another embodiment of the present invention, an audio systemincludes a digital audio signal source. The audio system furtherincludes a digital to analog converter (“DAC”) to (i) receive a digitalinput signal from the digital audio signal source, sampled at frequencyf_(S) and having frequency components primarily within a frequencybaseband, and out of baseband noise and to (ii) convert the digitalinput signal into a representative analog signal. The DAC includes amodulation circuit having a modulation frequency of approximately orequal to f_(S)/2 to modulate the digital input signal by approximatelyor equal to f_(S)/2 and a gain stage coupled to the modulation circuitto receive signals modulated by the modulation circuit and provide gainfor the DAC, wherein during operation the gain stage is associated withnoise having frequencies within the baseband. The DAC also includes ademodulation circuit, coupled to the gain stage and having ademodulation frequency equal to approximately or equal to f_(S)/2, todemodulate signals and allow frequency components of the input signal tofold back into the baseband, to reduce fold back into the baseband ofthe quantization noise frequency components, and modulate the noiseassociated with the gain stage substantially out of the baseband. Theaudio system also includes an amplifier coupled to the DAC and an audiooutput device coupled to the DAC.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention may be better understood, and its numerousobjects, features and advantages made apparent to those skilled in theart by referencing the accompanying drawings. The use of the samereference number throughout the several figures designates a like orsimilar element.

[0020]FIG. 1 (prior art) depicts an amplifier and chopping circuitry.

[0021]FIGS. 2A, 2B, and 2C depict signals associated with the amplifierand chopping circuitry of FIG. 1 in the frequency domain.

[0022]FIG. 3 depicts a digital signal processing system that includes adigital to analog converter with chopping circuitry operating atapproximately one-half of a digital input signal sampling frequency.

[0023]FIG. 4 depicts the digital signal processing system of FIG. 3 witha switched capacitor digital to analog converter having choppingcircuitry.

[0024]FIG. 5 depicts an embodiment of the switched capacitor digital toanalog converter of FIG. 4.

[0025]FIG. 6 depicts clock and control signals associated with theswitched capacitor digital to analog converter of FIG. 6.

[0026]FIG. 7 depicts a frequency response of the switched capacitordigital to analog converter of FIG. 6 at various points of reference.

[0027]FIG. 8 depicts the digital signal processing system of FIG. 3having a notch filter at a chopping frequency.

[0028]FIG. 9 depicts a modulator of the digital processing system ofFIG. 8 having a notch filter at a chopping frequency included in atransfer function of the modulator.

[0029]FIG. 10 depicts an embodiment of the modulator with the notchfilter of FIG. 9.

[0030]FIG. 11 depicts the digital processing system of FIG. 8 with apost-modulation, pre-DAC digital notch filter.

[0031]FIG. 12 depicts a frequency response of the notch filters of FIGS.10 and 11.

[0032]FIG. 13 depicts a frequency response of the output of the switchedcapacitor digital to analog converter of the system of FIG. 8 with andwithout notch filtering.

DETAILED DESCRIPTION

[0033] Signal processing technology operates on analog, digital andmixed signals (analog and digital). For example, audio signals,representing a signal of interest and having frequency componentsresiding within a baseband, begin as analog signals and are oftendigitized through sampling, and converted back into analog outputsignals using a digital-to-analog converter (DAC). Audio frequencyanalog output signals produce sound through a loud speaker, headphones,or other output device. Embodiments of the DAC system described hereinutilize chopping modulation technology to remove 1/f and other basebandnoise from a baseband of a signal of interest. In some embodiments, thechopping frequency is carefully selected to reduce noise fold back intothe baseband. Additionally, one or more notch filters are implemented toaddress parasitic modulation and associated noise fold back associatedwith non-ideal system properties, such as parasitic impedances. Forexample, a notch filter reduces the fold back of the noise into thebaseband from modulation at the chopping frequency and harmonicfrequencies thereof.

[0034] High performance digital signal processing systems, described inmore detail below, are particularly sensitive to noise, even low levelsof noise. In one embodiment, to address such noise concerns a digital toanalog converter (“DAC”) includes chopping circuitry. Chopping circuitryin general when combined with non-ideal (i.e. real-world) circuitcomponents includes as a byproduct the fold back of noise frequencycomponents into the baseband. Noise frequency components in a basebandof a signal of interest are generally undesirable. Chopping oftenresults in the fold back of such noise into a baseband of a digitalinput signal due to modulation caused by non-ideal properties of thedigital signal processing systems. In systems that generate noiseoutside the baseband, such as quantization noise, the folding back ofnoise into the baseband due to parasitic modulation can be verydetrimental to the quality of a signal of interest. This fold back ofnoise is a concern having no counterpart in conventional analog todigital converter technology.

[0035] A digital signal processing system that includes a DAC withchopping circuitry operating at a chopping frequency equal to orapproximately equal to one-half of a digital input signal samplingfrequency addresses noise fold back concerns due to parasitic modulationat twice the chopping frequency, 2f_(chop). Chopping at one-half thesampling frequency results in fold back into the baseband of frequencycomponents of the input signal itself, rather than noise components. Ina further embodiment, a notch filter inserts zeros into the frequencyresponse of the DAC at the chopping frequency and harmonics. The notchfilter is preferably designed to have an attenuation band at least equalto the baseband of an input signal of interest. The notch filter alsoassists in preventing fold back of noise into a baseband of the inputsignal of noise modulated at f_(chop). The various chopping signals arealso preferably coordinated to reduce the generation of additional noisewithin the system.

[0036]FIG. 3 depicts digital signal processing system 300. Digitalsignal processing system 300 represents one embodiment of a system thatbenefits from setting a chopping frequency f_(chop) equal to one-half asignal sampling frequency, f_(S) In one embodiment, digital signalprocessing system 300 is an audio system for receiving, processing, andproviding audio output signals. Input signal source 302 provides adigital input signal from any signal source, such as a compact diskplayer, MP3 player, audio/video system, or other audio recording and/orplayback device. An interpolation filter 304 performs oversamplingfunctions in a well-known manner on the digital input signal receivedfrom the input signal source 302. The interpolation filter 304 has asampling frequency, f_(S), provided by clock 308. The modulator-DAC 306processes the interpolated digital input signal to convert the digitalinput signal into an analog signal. The modulator-DAC 306 also includeschopping circuitry to modulate noise, such as 1/f noise out of abaseband of the digital input signal. Clock 308 provides various controlsignals to modulator-DAC 306 as described below. In general, clock 308provides a clock signal having a frequency of f_(S) to modulator-DAC 306and provides various chopping signals that have a frequency f_(chop)equal or approximately equal to f_(S)/2. As will become evident, it ispreferable that f_(chop) equal f_(S)/2 exactly. However, variations intiming are generally inevitable in real systems due to factors such asnon-ideal components and parasitic impedances. To avoid repeating“approximately” throughout this description unless otherwise indicated,references to “equal” in the context of f_(chop) equal f_(S)/2 mean“approximately equal or equal”.

[0037] A low pass filter 310, generally having a corner frequency at orabout the corner frequency, f_(B), of the baseband of the input signal,attenuates output signal frequency components outside the baseband. Afiltered output signal from low pass filter 310 is generally amplifiedbefore being provided to load 314, especially when driving a lowimpedance load. Thus, amplifier 312 amplifies the filtered output signaland provides the signal to load 314. Although load 314 may be any load,in audio applications, load 314 represents, for example, loud speakers,headphones, and other sound producing devices.

[0038]FIG. 4 depicts digital signal processing system 400, which is oneembodiment of the digital signal processing system 300. Themodulator-DAC 402 implements modulator-DAC 306 with a delta-sigmamodulator 404 and a switched capacitor DAC 406. In one embodiment, thedelta-sigma modulator 404 provides a series of multi-bit output signalsto switched capacitor DAC 406. The output signals of delta-sigmamodulator 404 have an average value over a predetermined time equal tothe digital input signal plus quantization error. “Delta-sigmamodulator” and “sigma-delta modulator,” a commonly used term, areinterchangeable terms for purposes of this specification. Thedelta-sigma modulator is clocked at f_(S). The frequency f_(S) isselected based upon the particular application of digital signalprocessing system 400. In audio applications, frequency f_(S) is, forexample, 6.144 MHz, which reflects a 128 bit word representation of anaudio signal originally sampled at 48 kHz. The frequency spectrum of thequantization noise generated by delta-sigma modulator 404 residesprimarily outside of an audio frequency baseband, approximately 0 to 25kHz. The quantization noise represents one example of noise that hasfrequency components that can fold back and mix with a signal ofinterest in the baseband when using conventional technology.

[0039]FIG. 5 depicts a multibit, switched capacitor DAC 500, which isone embodiment of switched capacitor DAC 406. Any DAC could substitutefor switched capacitor DAC 406. Switched capacitor DAC 500 includesmodulating chopping circuitry 502, which modulates the input signalprovided by delta-sigma modulator 404. Switched capacitor DAC 500includes demodulating chopping circuitry 508, which demodulates theoutput signal of gain stage G1.

[0040] Many well-known hardware, software, and hardware-softwarecombinations can be used to provide an input signal to switchedcapacitor DAC 500 that corresponds to an output signal of delta-sigmamodulator 404. In one embodiment, capacitor banks 504 and 506 eachcontain an identical number of “N” parallel coupled, correspondingsampling capacitors, collectively referred to as C_(S+) (504) and C_(S+)(506). Each sampling capacitor in capacitor bank 504 is charged inaccordance with one-bit of each N-bit word in the delta sigma modulator404 output signal. The same charging scheme applies to capacitor bank506. Capacitors associated with the same bit within capacitor banks 504and 506 are referred to herein as “corresponding capacitors”.

[0041] The switched capacitor DAC 500 operates in accordance with thecontrol signals depicted in FIG. 6. In this embodiment, all of thecontrol signals are generated with reference to an analog clock signal.The control signals, including chopping circuitry control signals, arepreferably phase-shifted relative to the analog clock signal andrelative to each other to allow settling of components and to minimizethe inclusion of switching noise into the signal being processed.Settling time is dependent upon specific component value and otherdesign considerations well-known to those of ordinary skill in the art.As will be described in more detail below, it has been identified thatwhile the inclusion of chopping circuitry 502 and 508 addresses some lowfrequency noise concerns (e.g. 1/f noise) within the baseband, a concernarises due, for example, to potential fold back into the baseband ofnoise, such as quantization noise, and consequential corruption of thesignal of interest due to parasitic modulation at frequencies f_(chop)and harmonics thereof and 2f_(chop) and harmonics thereof.

[0042] The operation of switched capacitor DAC 500 is now described inconjunction with the control signals depicted in FIG. 6. Beginning attime t0, after the analog clock rises and a sufficient time has beenallocated to allow circuits to settle, control signal φ₁ rises for asufficient time to cause switches 510 and 512 to conduct and short oneplate of C_(S)+ 504 and C_(S)− 506 sampling capacitors to the commonmode voltage Vcm. The sampling capacitors C_(S)+ 504 and C_(S−) 506 arecharged to Vdd through respective switches 518 and 522 or discharged toground GND, in accordance with the value of d* and {overscore (d)}*associated with each sampling capacitor, through respective switches 520and 524 while control signal φ₁ is HIGH. Each value of d represents theith bit of the N-bit output word, and {overscore (d)} represents thecomplement of d, 0≦i≦N-1, where N is the number of bits in an outputword of delta sigma modulator 404. Each of switches 518, 520, 522, and524 represent N respective switches, one for each of the N bits in thedelta sigma modulator 404 N-bit output word, where N is, for example,16.

[0043] After sampling capacitors C_(S+) 504 and C_(S−) 506 are chargedin accordance with the modulated output signal of delta sigma modulator404, control signal φ₂ causes switches 526, 528, 530 and 532 to conductand transfer charge to integrating capacitors C₁+514 and C₁ 516.

[0044] The operational phases of switched capacitor DAC 500 aresummarized in Table 1: TABLE 1 Control Signal ID and Switched CapacitorDAC State* 500 Operational Phase Φ₁ - HIGH Charge Φ₂ - HIGH Dump

[0045] Chopping circuitry 502 modulates the DAC input signal at inputterminals 534 and 536 and chopping circuitry 508 demodulates the outputsignal of gain stage G1. The chopping modulation and demodulationoperations are summarized in Table 2: TABLE 2 Chopping CircuitryChopping Circuitry Operational Phase Chopping Circuitry 502 ModulationChopping Circuitry 508 Demodulation

[0046] Chopping modulation and demodulation circuitry use direct andcross-over switches to modulate low frequency noise, such as 1/f noise,out of a signal baseband. Chopping circuits are particularly useful withlow frequency baseband applications such as audio applications. In oneembodiment, the direct switches coupled to respective input and outputterminals of a differential amplifier have nonoverlapping conductionphases with cross-over switches coupled to the terminals of thedifferential amplifier. Additionally, the direct switches coupled to therespective input and output terminals have different duty cycles withrespect to each other, and the cross-over switches coupled to therespective input and output terminals of the differential amplifier havedifferent duty cycles with respect to each other to reduce noisetransfer to an output of the system.

[0047] Referring to FIGS. 5 and 6, control signals φ_(chA) and φ_(chB)of chopping circuitry 502 are non-overlapping to prevent signalcorruption, preferably square waves (or approximations thereto), andcontrol conduction of switches 538 and 540, respectively. Choppingcircuitry 508 control signals φ_(chC) and φ_(chD) are alsonon-overlapping to prevent signal corruption, preferably square waves(or approximations thereto), and control conduction of switches 542 and544, respectively. A rise of control signal φ_(chA) at time t₃ causesdirect switches 538 to conduct and pass-through the delta-sigmamodulated input signal at input terminals 534 and 536 straight to gainstage G1 during the first depicted Dump phase occurring from t₄ to t₅.Direct switches 542 also conduct at time t₃, and, during the first Dumpphase gain stage G1 and pass-through an amplified output signal straightto the input terminals of gain stage G2. Control signal φ_(chC) falls att₆ followed by the fall of φ_(chA) at t₇ The delay between turningdirect switches 542 “off” (nonconducting) and turning direct switches538 off at the input of gain stage G1 is set by control signals φ_(chC)and φ_(chA) to provide a delay sufficient to prevent the switching noiseassociated with switches 538 from being passed through to the outputsV_(o)+ and V_(o)−. The delay time depends on the particularimplementation of the DAC and chopping circuitry. In one embodiment thedelay is on the order of 5 ns. Conduction and nonconduction of switches538, 540, 542, and 544 controlled by the edge of control signals orcontrolled by other properties of the control signals in a well-knownmanner.

[0048] At time t₈, prior to the beginning of the second depicted Dumpphase (t₉ through t₁₀), a rise of cross-over control signals φ_(chB)following the fall of φ_(chA) causes cross-over switches 540 to conduct.The delta-sigma modulated input signal at input terminals 534 and 536cross-over to opposite inputs of gain stage G1 during the second Dumpphase. Control signal φ_(chD) falls at t₁₁ followed by the fall ofφ_(chB) at t₁₂ again with a delay sufficient to prevent the switchingnoise associated with switches 540 from being passed through to theoutputs V_(o)+ and V_(o)−. When cross-over switches 544 conductbeginning at t₈, gain stage G1 provides an amplified output signalcrossed-over to the input terminals of gain stage G2. The demodulationphase of chopping circuit 508 ends at time t₁₂. At t₁₃, control signalφ_(chA) rises again, and the processes described above occurring betweent₃ and t₁₂ repeat during operation of switched capacitor DAC 500. Thetiming of control signals φ_(chA), φ_(chB), φ_(chC), and φ_(chD) areachieved, in one embodiment, by using the chop clock reference signal asa reference and using delay elements to achieve the desired timing.

[0049] “V_(o)+” and “V_(o)−” represent the respective differentialoutput signals of switched capacitor DAC 500. Gain stages G1 and G2 aredual output, fully differential operational amplifiers, which can be ofany design suitable for the application to which they are applied. Othergain stage implementations may be used. Other embodiments of switchedcapacitor DAC 500 use a single gain stage or more than two gain stages.

[0050] DACs often contain non-ideal properties such as parasiticcapacitances or timing errors. Such non-ideal properties can produceparasitic modulation, which can corrupt a signal of interest due to, forexample, fold back of noise into the baseband and mixing with the signalof interest. For example, the chopping circuitry 502 and 508 inassociation with various parasitic capacitances as well as othernon-ideal properties can cause noise to occur at f_(chop), 2f_(chop),and odd and harmonics of f_(chop) and 2f_(chop). For example, parasiticcapacitors C_(2p+) and C_(2p−) cause continuous time glitches to occurat the output of switched capacitor DAC 500 at a frequency of 2f_(chop)The parasitic modulation caused by the glitches results in fold backinto the baseband of signals at 2f_(chop) and harmonics. Additionally,parasitic modulation at frequency f_(chop) and harmonics thereof hasbeen observed to cause fold back of signals at f_(chop)=f_(S)/2. In oneembodiment, notch filters are added at parasitic modulation frequenciesto attenuate signals folded back into the baseband by parasiticmodulation at f_(chop).

[0051] Gain stage G2 also introduces 1/f noise. However, if the gain ofgain stage G1 is relatively high, the energy level of the gain stage G21/f noise is relatively low.

[0052]FIG. 7 depicts the frequency response of switched capacitor DAC500 and low pass filter 310 at various points of reference. Thefrequency axis and amplitudes are not to scale, and the depiction ofswitched capacitor DAC 500 has been simplified to highlight points ofreference in the frequency domain. Also, for clarity, FIG. 7 onlydepicts positive frequencies, and harmonics are not shown but areaffected in the same way as their fundamental frequencies. It isunderstood that a mirror image of the frequency spectrums in FIGS. 7 and13 reside at negative frequencies. In an audio application embodiment,the baseband of the input signal of interest from input signal source302 has a bandwidth ranging from 0 Hz to f_(B), where f_(B) is about 25kHz. At reference point A, frequency components of the input signal 702from the input signal source 302 reside within the baseband, andfrequency components of the quantization noise 704 from delta sigmamodulator 404 reside between frequencies f_(B) and f_(S) minus f_(B).The DAC input signal 706, received at input terminals 534 and 536,includes the input signal 702 and quantization noise 704. The spectrumis repeated at f_(S) and integer multiples of f_(S) in accordance withdigital sampling theory.

[0053] Referring to reference point B, chopping circuit 502 modulatesthe DAC input signal 706 at reference point A by frequency f_(chop),thereby transposing the frequency spectrum at reference point A up byfrequency f_(chop). Thus, the delta-sigma modulated input signal 702 nowresides within frequencies f_(chop)+/−f_(B) with a mirror image betweenf_(chop)−f_(B) and f_(chop), and some quantization noise 704 resideswithin the baseband. The 1/f noise 708, with higher energy levels in thebaseband, is introduced and shown at reference point C. Chopping circuit508 demodulates the signal at reference point C, thereby transposing thefrequency spectrum down by f_(chop) as depicted at reference point D.Thus, the modulated input signal 702 (the signal of interest) istransposed back to the baseband. During the chopping demodulation stage,the 1/f noise is modulated by f_(chop), thereby transposing the 1/fnoise frequency components up by f_(chop) as depicted at reference pointD.

[0054] As described above, parasitic modulation causes fold back intothe baseband of signals having frequencies within frequencies2f_(chop)+/−f_(B). By selecting f_(chop)=f_(S) /2, signals withinfrequencies f_(S)+/−f_(B) are folded back into the baseband. However,the signal of interest resides at f_(S)+/−f_(B). Thus, the signal ofinterest 702 is folded back and mixed with itself. Thus, although theremay be some gain change in the input signal, there is little or no noiseintroduced into the baseband due to 2f_(chop) modulation whenf_(chop)=f_(S)/2. The low pass filter 310 attenuates frequencycomponents in the output of gain stage G2 as depicted at reference pointE. As described above, non-ideal properties can cause parasiticmodulation at f_(chop) and harmonics. Referring to FIG. 7, quantizationnoise 704 resides at f_(chop). Therefore, any parasitic modulation atf_(chop) and harmonics causes fold back of quantization noise into thebaseband.

[0055] Referring to FIG. 8, modulator-notch filter-DAC 800 represents anembodiment of modulator-DAC 306 having a notch filter to attenuatefrequency components within f_(chop)+/−f_(B) prior to chopping. Thus,fold back of noise signals within frequencies f_(chop)+/−f_(B) into thebaseband is reduced in direct relation to the attenuation of noisewithin frequencies f_(chop)+/−f_(B).

[0056] Referring to FIGS. 10 and 11, the notch filter of modulator-notchfilter-DAC 800 is implemented within the digital signal processingsystem 400. The delta-sigma modulator 900 includes a conventionalsumming node 902 to sum a received input signal and the negative of thequantizer feedback output signal 1012. The delta-sigma modulator 1000also includes a first order main loop filter H(z) 904 for noise shaping.The main loop filter H(z) 1004 is modified by adding a parallel notchloop filter 1008. The notch loop filter 1008 has transfer functionsz⁻¹/(l+z⁻¹) with gain a1 and notch characteristics and 1/(1+z-⁻¹) withgain a2. Coefficients of the notch filter 1008 and gain are dependent onthe specific application of digital signal processing system 300 and aredetermined in a well-known manner to provide attenuation of frequencycomponents within frequencies f_(chop)+/−f_(B) and harmonics.

[0057]FIG. 11 depicts another embodiment of modulator-notch filter-DAC800 that includes a notch FIR filter 1102 implemented as a 2-tap FIRboxcar filter. Coefficients of notch FIR filter 1102 are also dependenton the specific application of digital signal processing system 300 andare determined in a well-known manner to provide attenuation offrequency components within frequencies f_(chop)+/−f_(B) and harmonics.It will be recognized by those of ordinary skill in the art that othernotch filter embodiments can be used to achieve attenuation atfrequencies f_(chop)+/−f_(B) and harmonics thereof, and, thus, preventnoise otherwise present at f_(chop)+/−f_(B) and harmonics thereof fromfolding into the baseband and mixing with the signal of interest.

[0058]FIG. 12 depicts an actual frequency spectrum plot of oneembodiment of notch loop filter 908 and notch FIR filter 1102 in anaudio application, where f_(S)=6.144 MHz, f_(B)=25 kHz, andf_(chop)=f_(S)/2.

[0059]FIG. 13 depicts at various points of reference the frequencyresponse of switched capacitor DAC 500 in conjunction with a notchfilter of modulator-notch filter-DAC 800 attenuating frequencycomponents within frequencies f_(chop)+/−f_(B) and harmonics thereof andchopping at frequency f_(chop)=f_(S)/2. The frequency and amplitude axesare not to scale, and the depiction of switched capacitor DAC 500 hasbeen simplified to highlight points of reference in the frequencydomain. Also, for clarity FIG. 13 only depicts positive frequencies, andharmonics are not shown but are affected in the same away as theirfundamental frequencies. In an audio application embodiment, thebaseband of the input signal of interest from input signal source 302has a bandwidth ranging from 0 Hz to f_(B), where f_(B) is about 25 kHz.

[0060]FIG. 13 depicts the effects of parasitic fold back of noise atfrequency f_(chop), with and without an attenuation band centered atf_(chop). Chopping events at reference points A, B, C, and E occur aspreviously described with respect to FIG. 7. As described above,nonideal properties associated with DACs, such as switched capacitor DAC500, can result in parasitic modulation at the chopping frequencyf_(chop) and harmonics and at 2f_(chop) and harmonics. The parasiticmodulation at frequencies f_(chop) and 2f_(chop) cause signals havingfrequencies within f_(chop)+/−f_(B) and 2f_(chop)+/−f_(B) to fold intothe baseband and mix with the signal of interest 702. The frequencyspectrum 1302 illustrates the fold back of quantization noise 704 intothe baseband in the absence of signal attenuation at frequencies withinf_(chop)+/−f_(B). The folded back quantization noise mixes with thesignal of interest 702 and, thus, corrupts the signal of interest 702.

[0061] The frequency spectrum 1304 illustrates the attenuation ofquantization noise 704 within frequencies f_(chop)+/−f_(B) due to thepresence of a notch filter, such as notch loop filter 1008 or notch FIRfilter 1102. The attenuation of noise within frequenciesf_(chop)+/−f_(B) reduces or prevents, depending on the degree ofattenuation and attenuation bandwidth, fold back of noise due toparasitic modulation at frequency f_(chop). As described above,corruption of the signal of interest 702 due to folding at 2f_(chop) isavoided by setting f_(chop)=f_(S)/2 so that the signal of interest 702is folded back upon itself. Notches can also be set at harmonics off_(chop) to prevent fold back of noise into the baseband. Furthermore,notch filters can be used to attenuate noise prior to chopping at anyother parasitic modulation frequency or frequencies to prevent fold backof noise into the baseband during chopping.

[0062] Thus, the signal processing system herein provides significantnoise attenuation within a baseband of interest using carefully selectedchopping frequencies and/or notch filters with attenuation bands aroundparasitic modulation frequencies.

[0063] The signal processing systems disclosed herein can bemanufactured using well-known integrated, discrete, or a combination ofintegrated and discrete components. Additionally software in combinationwith a processor can be used to implement features of the signalprocessing systems, such as a notch filter. Those of ordinary skill inthe art will recognize that the signal processing systems disclosedherein can be implemented with a wide range of components other thanthose disclosed herein. For example, the digital signal modulators couldbe implemented using mixed signal (analog and digital) technology.Additionally, the attenuation bands of notch filters can be set togreater than, less than, or equal to f_(B) depending on, for example,the amount of acceptable noise fold back into the baseband.Additionally, many implementations of the DAC are possible includingusing a single bit delta-sigma modulator in place of the multi-bitdelta-sigma modulator. The multi-bit switched capacitor circuit canremain a multi-bit circuit with an accumulation of bits from the singlebit delta-sigma modulator or could be a single bit circuit.

[0064] Although the present invention has been described in detail, itshould be understood that various changes, substitutions and alterationscan be made hereto without departing from the spirit and scope of theinvention as defined by the appended claims. For example, application ofthe signal processing systems described herein is not limited to audioapplications.

What is claimed is:
 1. A digital to analog converter system, comprising:a digital to analog converter (“DAC”) to (i) receive a digital inputsignal, sampled at frequency f_(S) and having frequency componentsprimarily within a frequency baseband, and out of baseband noise and to(ii) convert the digital input signal into a representative analogsignal, the DAC comprising: a modulation circuit having a modulationfrequency of approximately or equal to f_(S)/2 to modulate the digitalinput signal by approximately or equal to f_(S)/2; a gain stage coupledto the modulation circuit to receive signals modulated by the modulationcircuit and provide gain for the DAC, wherein during operation the gainstage is associated with noise having frequencies within the baseband;and a demodulation circuit, coupled to the gain stage and having ademodulation frequency equal to approximately or equal to f_(S)/2, todemodulate signals and allow frequency components of the input signal tofold back into the baseband, to reduce fold back into the baseband ofthe quantization noise frequency components, and modulate the noiseassociated with the gain stage substantially out of the baseband.
 2. Thedigital to analog converter system of claim 1 further comprising: amodulator, coupled to the DAC, to receive a first digital input signalsampled at a frequency, f_(S), to modulate the digital input signal,wherein the first digital input signal includes frequency componentsresiding within the baseband bounded by frequencies f₁ and f₂ andwherein a byproduct of the first digital input signal modulationincludes quantization noise having frequency components residing outsideof the baseband, wherein the input signal of the DAC is derived from themodulated first digital input signal.
 3. The digital to analog convertersystem of claim 2 wherein f₁ and f₂ define respective high and lowfrequencies of an audio signal baseband.
 4. The digital to analogconverter system of claim 3 wherein f₁ is approximately or equal to 0 Hzand f₂ is approximately or equal to 25 kHz.
 5. The digital to analogconverter system of claim 2 wherein the modulator is a delta sigma typemodulator, the system further comprising: a notch filter implemented ina feedback loop of the modulator and having a frequency attenuation bandto attenuate signals having frequencies within a band folded back intothe baseband by parasitic modulation.
 6. The digital to analog convertersystem of claim 5 wherein the frequency attenuation band equals +/−abandwidth of the frequency baseband.
 7. The digital to analog convertersystem of claim 5 wherein the frequency attenuation band is centered atf_(S)/2.
 8. The digital to analog converter system of claim 1 whereinthe modulation circuit is a chopping modulator comprising series andcross-over switches to chop input signals of the gain stage, thedemodulation circuit is a chopping circuit comprising series andcross-over switches to chop output signals of the gain stage, and thegain stage is a component of a fully differential amplifier.
 9. Thedigital to analog converter system of claim 1 wherein the DAC comprisesa switched capacitor digital to analog converter.
 10. The digital toanalog converter system of claim 9 wherein the switched capacitordigital to analog converter includes sampling capacitors to receivecharge representative of the input signal, a feed back capacitor, andswitches to share charge between the sampling capacitors and the feedback capacitor.
 11. The digital to analog converter system of claim 1wherein the digital input signal includes audio frequency components.12. The digital to analog converter system of claim 1 furthercomprising: a notch filter to receive the modulated digital input signaland to provide a filtered output signal to the DAC, the notch filterhaving a frequency attenuation band to attenuate signals havingfrequencies within a band folded back into the baseband by parasiticmodulation.
 13. The digital to analog converter system of claim 12wherein the frequency attenuation band equals +/−a bandwidth of thefrequency baseband and is centered at f_(S)/2.
 14. The digital to analogconverter system of claim 12 wherein the filter is a digital finiteimpulse response filter implemented using hardware components.
 15. Thedigital to analog converter system of claim 1: wherein the modulationcircuit further comprises: input terminals to receive the digital inputsignal; and direct and cross-over switches coupled to alternatelypass-through and cross-over a differential input signal in accordancewith pass-through control signal φ_(chA) and cross-over control signalφ_(chB), wherein control signal φ_(chA) is out-of-phase with controlsignal φ_(chB); and the demodulation circuit further comprises: inputterminals to receive an output signal of the gain stage and alternatelypass-through and cross-over the differential input signal modulated bythe modulation circuit in accordance with pass-through control signalsφ_(chC) and cross-over control signal φ_(chD), wherein control signalφ_(chD) is out-of-phase with φ_(chB), and control signals φ_(chA) andφ_(chC) are nonoverlapping with respect to control signals φ_(chB) andφ_(chD).
 16. The digital to analog converter system of claim 15 whereincontrol signal φ_(chC) is configured to cause the direct switchescontrolled by φ_(chC) to become nonconductive prior to nonconduction ofthe direct switches controlled by φ_(chA), and φ_(chD) is configured tocause the direct switches controlled by φ_(chD) to become nonconductiveprior to nonconduction of the direct switches controlled by φ_(chB). 17.The digital to analog converter system of claim 15 wherein controlsignals φ_(chA), φ_(chB), φ_(chC), and φ_(chD) are square waves.
 18. Amethod of converting a digital input signal, sampled at a frequencyf_(S) and having frequencies within a baseband, into a representativeanalog signal, modulating out of the baseband low frequency noisecomponents associated with a digital to analog converter (“DAC”) gainstage, and reducing fold back of quantization noise frequency componentsresiding outside the baseband, the method comprising: modulating a DACinput signal, which is derived from the digital input signal, and thequantization noise by a modulation frequency of approximately or equalto f_(S)/2; providing the modulated DAC input signal to the DAC gainstage; and demodulating output signals of the DAC gain stage by ademodulation frequency of approximately or equal to f_(S)/2 to allowfrequency components of the input signal to fold back into the baseband,to reduce fold back into the baseband of the quantization noisefrequency components, and modulate the noise associated with the gainstage substantially out of the baseband.
 19. The method as in claim 18wherein modulating the DAC input signal generates a noise signal havingfrequency components centered at f_(S)/2, the method further comprising:filtering the modulated digital input signal to attenuate noise signalcomponents having frequencies at f_(S)/2+/−f_(B), wherein f_(B) is acorner frequency of a bandwidth of the baseband.
 20. The method as inclaim 19 wherein filtering the modulated digital signal furthercomprises: filtering the digital input signal using a digital finiteinput response notch filter.
 21. The method as in claim 18 furthercomprising: modulating the digital input signal, which generates thequantization noise as a byproduct of modulating the input signal. 22.The method as in claim 21 wherein modulating the DAC input signalgenerates a noise signal having frequency components centered atf_(S)/2, the method further comprising: modulating the digital inputsignal comprises modulating the digital input signal using a delta sigmamodulator; and filtering the modulated digital input signal to attenuatesignal components having frequencies of approximately f_(S)/2+/−f_(B),wherein f_(B) is a corner frequency of a bandwidth of the baseband,wherein filtering the modulated digital signal further comprisesfiltering the modulated digital input signal within a feedback loop ofthe delta sigma modulator.
 23. The method of claim 18 wherein thedigital input signal is an audio signal.
 24. The method of claim 18wherein: modulating the DAC input signal further comprises alternatelypassing-through and crossing-over the DAC input signal in accordancewith pass-through control signal φ_(chA) and cross-over control signalφ_(chB), wherein control signal φ_(chA) is out-of-phase with controlsignal φ_(chB); and demodulating output signals of the DAC gain stagefurther comprises alternately passing-through and crossing-over the DACinput signal modulated by the modulation circuit in accordance withpass-through control signals φ_(chC) and cross-over control signalφ_(chD), wherein control signal φ_(chD) is out-of-phase with φ_(chB),and control signals φ_(cbA) and φ_(chC) are nonoverlapping with respectto control signals φ_(chB) and φ_(chD).
 25. The method of claim 24further comprising: ceasing the pass-through of the DAC input signal inaccordance with control signal φ_(chC) prior to ceasing the pass-throughof the DAC input signal in accordance with control signal φ_(chA); andceasing the crossing-over of the DAC input signal in accordance withcontrol signal φ_(chD) prior to ceasing the crossing-over of the DACinput signal in accordance with control signal φ_(chB).
 26. An audiosystem comprising: a digital audio signal source; a digital to analogconverter (“DAC”) to (i) receive a digital input signal from the digitalaudio signal source, sampled at frequency f_(S) and having frequencycomponents primarily within a frequency baseband, and out of basebandnoise and to (ii) convert the digital input signal into a representativeanalog signal, the DAC comprising: a modulation circuit having amodulation frequency of approximately or equal to f_(S)/2 to modulatethe digital input signal by approximately or equal to f_(S)/2; a gainstage coupled to the modulation circuit to receive signals modulated bythe modulation circuit and provide gain for the DAC, wherein duringoperation the gain stage is associated with noise having frequencieswithin the baseband; and a demodulation circuit, coupled to the gainstage and having a demodulation frequency equal to approximately orequal to f_(S)/2, to demodulate signals and allow frequency componentsof the input signal to fold back into the baseband, to reduce fold backinto the baseband of the quantization noise frequency components, andmodulate the noise associated with the gain stage substantially out ofthe baseband; an amplifier coupled to the DAC; and an audio outputdevice coupled to the DAC.
 27. The audio system of claim 26 wherein themodulation circuit is a chopping modulator comprising series andcross-over switches to chop input signals of the gain stage, thedemodulation circuit is a chopping circuit comprising series andcross-over switches to chop output signals of the gain stage, and thegain stage is a component of a fully differential amplifier.
 28. Theaudio system of claim 26 further comprising: a notch filter to receivethe modulated digital input signal and to provide a filtered outputsignal to the DAC, the notch filter having a frequency attenuation bandto attenuate signals having frequencies within a band folded back intothe baseband by parasitic modulation.
 29. The digital to analogconverter system of claim 28 wherein the frequency attenuation bandequals +/−a bandwidth of the frequency baseband and is centered atf_(S)/2.